1. Field of the Invention
The present invention relates generally to center pivot irrigation systems that employ a plurality of wheeled support towers secured to and spaced along the length of a water distribution pipe. In particular, the present invention relates to a center pivot irrigation system having an improved worm gear wheel drive assembly for driving the wheels of the support towers for moving the water distribution pipe around a central pivot axis of the irrigation system.
2. Description of the Related Art
Pivot irrigation systems are widely used throughout the world to provide water for agricultural purposes in arid regions. Typically, such systems include a plurality of spaced support towers connected by truss sections that support an elevated water distribution pipe between the towers. The water distribution pipe extends radially from a central pivot communicating with a pressurized water supply. Water passing through the distribution pipe is forced out through a number of sprinkler heads, spray guns, drop nozzles, and the like, spaced along the length of the pipe. Each tower in the system is supported on wheels that are power rotated at slow speeds to move the tower in a circular path about the central pivot to thereby irrigate a generally circular tract of land.
A number of drive assemblies have been developed for driving the support wheels of pivot irrigation systems. The most widely used drive assembly in the industry today is a worm gear drive developed by the present applicant, which is described in the Applicant's prior U.S. Pat. No. 3,623,662. The conventional worm gear drive consists of a drive motor and center gearbox having front and rear output shafts with a worm pinion mounted on the front and rear ends of the output shafts. A worm wheel is carried by the hub of each support wheel and is placed in meshing engagement with a respective worm pinion mounted on the output shafts. The worm wheel is thus driven by the worm pinion upon rotation of the output shaft, thereby causing the support wheel to rotate and move the respective tower of the irrigation system. The conventional worm gear drive provides a 50:1 gear reduction ratio between the output shaft of the drive motor and center gearbox and the hub of the support wheel.
The conventional worm gear drive assembly disclosed in the Applicant's U.S. Pat. No. 3,623,662, provided a simple, dependable, and relatively inexpensive construction. Moreover, the conventional worm gear drive assembly provided a significant advantage over earlier drive arrangements in that the meshing of the worm pinion with the worm wheel prevented the driven support wheels from rolling freely forward or backward as the irrigation system traverses uneven terrain. These advantages were quickly recognized throughout the industry and have made the conventional worm gear drive disclosed in the Applicant's U.S. Pat. No. 3,623,662 the standard and preferred drive assembly for center pivot irrigation systems for more than two decades.
However, the conventional worm gear drive assembly is quite inefficient in transferring power from the drive motor to the driven wheels. For example, the conventional worm gear arrangement operating at slow speeds has an approximate overall efficiency of only 40% due to the friction loss between the worm gear and the worm pinion. The efficiency of the worm gear drive assembly is particularly low in irrigation systems that require intermittent operation of the drive wheels due to the higher static coefficient of friction between the worm gear and the worm pinion. This inefficiency in the drive assembly has resulted in an increased horsepower and energy consumption required to obtain the desired torque and speed for moving the support towers of the pivot irrigation system.
A variety of other gear drive arrangements have been developed for center pivot irrigation systems. However, due to various problems with the construction and operation of these other systems, they have met with little acceptance in the industry.
For example, U.S. Pat. No. 4,093,036, issued to Knutson, discloses a tower drive assembly that uses three high ratio gear sets between a drive motor and each support wheel. The high ratio gear sets each include a first gear mounted on a support shaft and having a plurality of generally parallel rollers supported from and spaced equally about and radially outwardly from the support shaft. The gear sets each include a second gear mounted on a support shaft and having a plurality of outwardly opening notches formed in and spaced about an outer peripheral portion. The rollers of the first gear are meshed with the notches of the second gear of each set.
The successive high ratio gear reductions of the tower drive assembly of U.S. Pat. No. 4,093,036 permit the drive motor to operate at relatively low torques, thereby lessening the tendency of surges in the driving torque applied to the drive wheels. However, this tower drive assembly is complex and expensive to manufacture due to the roller assemblies required for each gear set. Moreover, unlike the worm gear drive arrangement of the Applicant's U.S. Pat. No. 3,623,662, the tower drive assembly of U.S. Pat. No. 4,093,036 would not effectively prevent free rolling of the drive wheels when the support towers traverse uneven terrain. A drive motor with a braking arrangement would normally be required for this purpose. However, such motor braking arrangements have been found unsuitable for the rugged operating conditions encountered by the intermittent operation of center pivot irrigation systems in uneven terrain.